Stress-induced martensitic transformation

Monl] Monzen, R., Kato, M., Stress-Induced Martensitic Transformation of Small Co-Fe Particles in a Cu Matrix , ISIJ International, 33(8), 898-902 (1993) (Abstract, Experimental, Morphology, 23) cited from abstract... 

R. P, Reed, G. J. Guntner, and R. L. Greeson, "On the Reduction in Te isile Strength and the Stress-Induced Martensitic Transformation of Some Austenitic Stainless Steels at very Low Temperatures, to be presented at the October, 1960 AIME in Philadelphia. 

The deformation of alloys can be facilitated if thermally induced or stress-induced transformations of one kind or another take place during the time that stress is being appKed to the sample. Either high-temperature or low-temperature transformations are eligible for consideration in this section, which draws implicitly on the contents of Sections 5.2 and 5.3.4 (stress-induced martensitic transformation and twinning) and Sections 8.3 and 12.14 (low-temperature and high-temperature serrated yielding) as well as other soxirces. 

Stress induced martensitic transformation is a transformation firom one ciystallographic form to another form and associated with a displacement of chains to new positions in the new crystallographic ceU in order to acconunodate the deformation. An example of martensitic transformation firom orthorhombic to monoclinic form was found in oriented polyethylene with well defined texture subjected to uniaxial compression. Martensitic transformation was also found in other polymers in poly(L-lactic acid) [119] and in nylon 6 with the a-form transforming to the 7-form [120,121]. 

Stress induced martensitic transformation and twinning alone are not responsible for large strain deformation. 

Another property pecuHar to SMAs is the abiUty under certain conditions to exhibit superelastic behavior, also given the name linear superelasticity. This is distinguished from the pseudoelastic behavior, SIM. Many of the martensitic alloys, when deformed well beyond the point where the initial single coalesced martensite has formed, exhibit a stress-induced martensite-to-martensite transformation. In this mode of deformation, strain recovery occurs through the release of stress, not by a temperature-induced phase change, and recoverable strains in excess of 15% have been observed. This behavior has been exploited for medical devices. 

Most of the austenitic stainless steels are known to undergo a strain-induced martensitic transformation [1], Aless well-known fact is that certain commercial grades of AISI 304 and AISI 304L also undergo spontaneous transformation upon quenching to 76°K [2]. This report will be confined to the mechanical properties of alloys that undergo strain-induced transformation only. The strain-induced martensitic transformation is dependent on the temperature of deformation and the nature of the applied stress. A treatment of one theory of strain-induced martensitic transformation may be found in the work of Patel and Cohen [3]. 

Stress-Induced Martensite. Martensitic transformation can also be induced in a quenched metastable P-phase by an externally appHed stress. The structxare of the stress-induced (or assisted) martensitic products has been reported to be fee (or fet), hep, and orthorhombic (distorted hep). However, previoiisly reported hexagonal (aO stress-induced martensites were orthorhombic (a") and that the misinterpretation arose fi t)m the overlap of a and a" reflections. The triggering stress to induce P-to-a" transformation is as low as 150 MPa (22 ksi). Hydrogen, a P-phase stabilizer, tends to increase the triggering stress. 

Fig. 10.27. Schematic of biaxial loading apparatus used to examine stress-induced transformations and resulting microstructures in martensites (courtesy of R. James).

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